[Rd] Return values from .Call and garbage collection [Additional information added]

[Duncan Murdoch]

On 27/01/2009 7:11 AM, Jon Senior wrote:
> Hi all,
> 
> I'm posting this here as it discusses an issue with an external C library. If it would be better in R-Help, then I'll repost.
> 
> I'm using an external library which I've written, which provides a large set of data (>500MB in a highly condensed format) and the tools to return values from the data. The functionality has been tested call by call and using valgrind and works fine, with no memory leaks. After retrieval, I process the data in R. A specific function is causing a problem that appears to be related to the garbage collector (judging by symptoms).
> 
> In the C code, a Matrix is created using
> 
> PROTECT(retVal = allocMatrix(INTSXP, x, y));
> 
> Values are written into this matrix using
> 
> INTEGER(retVal)[translatedOffset]=z;
> 
> where "translatedOffset" is a conversion from a row/column pair to an offset as shown in R-exts.pdf.
> 
> The last two lines of the function call are:
> 
> UNPROTECT(1);
> return retVal;
> 
> The shared library was compiled with R CMD SHLIB and is called using .Call.
> 
> Which returns our completed SEXP object to R where processing continues.
> 
> In R, we continue to process the data, replacing -1s with NAs (I couldn't find a way to do that in that would make it back into R), sorting it, and trimming it. All of these operations are carried out on the original data.
> 
> If I carry out the processing step by step from the interpreter, everything is fine and the data comes out how I would expect. But when I run the R code to carry out those steps, every now and again (Around 1/5th of the time), the returned data is garbage. I'm expecting to receive a bias per iteration that should be -5 <= bias <= 5, but for the garbaged data, I'm getting results of the order of 100s of thousands out (eg. -220627.7). If I call the routine which carries out the processing for one iteration from the intepreter, sometimes I get the correct data, sometimes (with the same frequency) I get garbage.
> 
> There are two possibilities that I can envisage.
> 1) Race condition: R is starting to execute the R code after the .Call before the .Call has returned, thus the data is corrupted.
> 2) Garbage collector: the GC is collecting my data between the UNPROTECT(1); call and the assignment to an R variable.
> 
> The created matrices can be large (where x > 1000, y > 100000), but the garbage doesn't appear to be related to the size of the matrix.
> 
> R version 2.8.1 (2008-12-22), running on Fedora 10 on a Centrino dual-core with 3GB of RAM.
> 
> Any ideas what steps I could take to proceed with this? Or other possibilities than those I've suggested? For reasons of confidentiality I'm unable to release test code, and the large dataset might make testing difficult.

This sounds like a situation where gctorture() would help a lot.  After 
gctorture(TRUE), R will do a garbage collection before every allocation, 
so gc related errors will be much more likely to surface. As long as 
most of the work of your code is happening in C, this won't slow things 
down to be impossibly slow, as it sometimes does when all the work is in R.

It won't tell you where the error is, but making it happen reproducibly 
should help with that.

Duncan Murdoch